The present invention relates to a maintenance method in a nuclear power plant, and more particularly, to a maintenance method for maintaining a suppression chamber and a spent fuel storage pool provided in a nuclear reactor building in a nuclear power plant.
The reactor building of a nuclear power plant is provided with a reactor primary containment vessel (called containment vessel hereinlater) to enclose radioactive materials leaking from a reactor core within a nuclear reactor primary system and to prevent leakage of radiation in the event of a reactor failure accident in the primary system.
All of the containment vessels provided in boiling-water reactors (BWR), including advanced boiling-water reactors (ABWR), are pressure-suppression type vessels, which are generally constructed to be provided with a dry well and a suppression chamber.
FIG. 10 is a schematic cross-sectional view showing one example of such a containment vessel in the boiling-water reactor mentioned above. A primary containment vessel 1 is provided with a dry well 2 and a suppression chamber 3, and a reactor pressure vessel 5 is arranged at the central portion of the interior of the dry well 2 and supported by a reactor pressure vessel pedestal 4. A suppression pool 6 is provided in the suppression chamber 3 and is always filled with water.
The dry well 2 and the suppression chamber 3 communicate with each other through a vent pipe 7. The vent pipe 7 is connected to a downcomer 8 within the suppression chamber 3. The downcomer 8 is opened at a tip end portion to the water of the suppression pool 6.
FIG. 11 is a schematic cross-sectional view showing a primary containment vessel in a boiling-water reactor different in type from that of FIG. 10. As in the case of FIG. 10, the containment vessel of FIG. 11 is provided with a dry well 2 and a suppression chamber 3. A reactor pressure vessel 5 is provided at the central portion of the interior of the dry well 2 and is supported by a reactor pressure vessel pedestal 4. A suppression pool 6 is provided in the suppression chamber 3 and is always filled with water.
The dry well 2 and the suppression chamber 3 communicate with each other through a vent pipe 7. The vent pipe 7 has openings at forked tip end portions into the water of the suppression pool 6 in the suppression chamber 3.
The suppression chambers 3 shown in FIGS. 10 and 11 are formed by using steel plates. Since the steel plate does not have a corrosion allowance, the surface thereof is coated in viewpoints of corrosion resistance, water resistance and decontamination. Therefore, on the basis of the idea of preventive maintenance that the suppression chamber 3 is repaired before the life of a coated film applied onto the steel plate of the suppression chamber is over, the coated film of the suppression chamber 3 must be repaired almost at ten years intervals.
In the case of performing such repair coating operation, the state of the coated film on the inner surface of a suppression pool wall 6a is conventionally inspected throughout the pool by using a remote-controlled underwater camera or the like provided within the suppression chamber 3 in advance. Based on the inspection result, re-coating timing and re-coating areas must be determined. In the re-coating operation, first, the suppression chamber 3 is drained off, and the suppression pool 6 is made vacant by draining off the chamber 3.
In this state, although the re-coating operation is conducted. In the air, unlike in the water, radiation shielding effect is reduced or lost. For this reason, it is required to carry out the decontamination for removing radioactive materials present in the suppression chamber 3 before the re-coating operation. Since the radioactive materials involve substances or matters floating in the water of the suppression pool or those deposited on the bottom thereof which exist as residues or sludges, the inner surface of the suppression pool wall 6a is washed or the floating substances and/or sludges are removed through the remote control operation as the decontamination operation.
After the decontamination is over, a scaffold is mounted, operators go downs along the suppression chamber wall 6a through the scaffold, carries out substrate treatment for the target coated film to be repaired and then starts re-coating.
After all of the repair target surfaces are re-coated, the scaffold is dismounted and the suppression pool is filled up with water again, thereby completing the operation.
Further, as shown in FIG. 12, a spent fuel storage pool 10 is provided in the nuclear reactor building 9 for storing the spent fuel, which was burnt in the reactor of the containment vessel 1 and which life has expired. Since the interior of the spent fuel storage pool 10 is normally lined with a stainless steel, it is not necessary to apply coating. However, in view of the deterioration of the lining and a generation of other various deposits, an internal inspection is desired. Conventionally, when inspecting and decontaminating the spent fuel storage pool 10, the pool is drained off.
However, it takes considerable workload, time and cost to drain off the suppression chamber 3 and the spent fuel storage pool 10 and to perform decontamination following the above-stated internal inspection and repair coating operation. Further, if the decontamination operation is performed while the suppression chamber 3 and the spent fuel storage pool 10 are being drained off, i.e., in the air, it requires more operators due to the fact that radiation dose in the air becomes larger than that in the water, which also requires far more facility, considerable labor, time and cost.
Moreover, if the repair coating operation is conducted for the local deterioration of the coated film within the suppression chamber 3, it is required to entirely drain off the suppression pool 6. It takes, therefore, considerable workload, time and cost to perform such a local repair.